Ion acceleration apparatus for coincidence time-of-flight mass specrometers



D 29, 1964 A. 1.. WAHRHAFTIG ETAL 3,163,752

ION ACCELERATION APPARATUS FOR comcwzucs TIME-OF-FLIGHT MASS SPECTROMETERS Filed Aug. 20. 1962 5 o 8 E d a 9 L -jF- NA? g\ 8\ Us V, .l H- T a" g N v- =1 L 5% a; 5 7

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AUSTIN L. WAHRHAFTIG MARVIN L VESTAL y MANFRED KRAUSE W|LLIAM H. JOHNSTON A TTORNEYS' United States Patent ION ACCELERATIGN AlPARATUS FUR C(HNQI- DENCE T-OF-FLIGi-IT MASS SPECTROM- ETERS Austin L. Wahrhaftig, Salt Lake City, Utah, and Marvin L. Vestal, Sparks, and Manfred Kranse and William H. Johnston, Baltimore, Md, assignors to William H. Johnston Laboratories, Inc., Baltimore, Md, a corporation of Maryland Filed Aug. 20, 1962, Ser. No. 217,8ti6 10 Claims. (Cl. 250-419) This invention relates to apparatus for ion acceleration and, in particular, to an apparatus for accelerating ions between a source and a detector in such fashion as to achieve good ion resolution with higher efiiciency than normally obtained.

Particularly for use in mass spectrometers, it is necessary to accelerate and focus ions obtained from a source and directed to an ion detector. In time-ot-flight mass spectrometers it is important that ions of the same masses reach the detector at the same time, even though they may have been generated at slightly different distances from the detector and with diiferent initial velocities directed in dififerent directions. One system for accelerating and focusing ions between a source and an ion detector involves use of aperture electronor ion lenses, with a fairly low field intensity in the area between the ion source and the first lens, and a relatively higher field intensity between the first and a second lens. This scheme is advantageous in that ions passing through the lenses to the detector tend to arrive at the detector at the same time if they are of the same mass, despite small differences in initial velocity and differences in distance from the detector. This is particularly true when a long drift tube, or field-free region is employed between the lenses and the detector.

The aperture lenses of this prior art suggestion are normally formed by metal plates having apertures of appropriate rectangular shape therethrough, but, in order to minimize the perturbing efiect of the apertures on the electric fields between the plates, it has become customary to cover the apertures with fine Wire mesh of high transparency. Unfortunately, however, the electric field in the vicinity of each wire of the mesh is highly non-uniform, so that each hole in the mesh, separating regions of different field strengths, acts as a lens causing the beam of ions passing through the hole to be brought to a'focus at a short distance beyond the mesh. Because of the focal length of such ion lenses, the ion beam diverges beyond each focus within a cone having arela-tively large apex angle. Consequently, a very large percentage of the ions passing through each aperture impinge upon the walls of the lens plates between that aperture and the detector, and cannot be available for detection. In applications of the mass spectrometer in which efiiciency of ion counting is unimportant, this may be satisfactory, but in other applications it is desirable to deliver to the detector as many as possible of the ions which are actually created by the source. This is true, for instance, in the coincidence mass spectrometer disclosed in Rosenstock Patent No. 2,999,157, issued Sept. 5, 1961.

It is a primary object of this invention to deliver to the ion detector the ions abstracted from the source with both high resolution and higher efiiciency than obtained by the apparatus of the prior art.

Particularly in the application of this invention to the coincidence mass spectrometer, wherein both electrons and positive ions are detected, it is desirable to collect electrons with high efficiency from the ionization region and it is a further object of this invention to provide for such collection.

3,153,752 Patented Dec. 29., 1964 "ice The above and other objects of the invention, as will be apparent from the description to follow, are achieved by providing a pair of electron lenses between the ion source and the detector, with the first lens being slightly diverging in operation so that the ions reach the second lens appearing as if they originated from a virtual focus farther away from the second lens than is the ion source. The second lens is convergent in action upon the ion beam and is operative to change the diverging beam into 'a substantially parallel beam which is supplied to the detector.

While the present invention employs a relatively low field intensity between the first lens plate and the ioniz-ation region, and a relatively high field intensity farther away from the ionization region, an intermediate apertured plate is also provided and supplied with a voltage such that the field intensity between it and the first plate is less than the field intensity between the first plate and the ionization region. As a result, the ion beam is initially divergent as it passes through the first plate and is then converged to some extent as it passes through the second plate so that it would only be slightly divergent when it arrives at the convergent lens.

The same feature as indicated above, namely the use of the lower field intensity between a first and a second apertured plate than between the ionization region and the first plate, is employed in the accelerator between the ionization region and the electron detector.

The invention will now be more fully described in conjunction with a drawing showing a preferred embodiment thereof.

in the drawing, the single figure is a diagrammatic illustration of the apparatus of the invention.

The ionization region which is operative to supply ions (electrons and positive ions) to the detectors, is shown generally at 10. This region functions as a source of ions which may be obtained by passing a beam of ionizing radiation through the gas-filled area between apertured metal plates 11 and 12 at the opposite sides of the ionization region 10. The ion source itself is not at all critical to the present invention and it may be of the same type as indicated in Rosenstock Patent 2,999,157.

It will be evident that ionization events are caused all along the path of the ionizing radiation but the plates 11 and 12 are effective in blocking positive ions and electrons, respectively, from all areas of the space between the plates, except those opposite the apertures 13 and 14, respectively, in the plates.

The objective 'of the apparatus of the invention is of course to' urge positive ions through the aperture 13 in plate 11 to a positive ion detector 15 shown at the'left of FIG. 1, and to urge electrons through the aperture 14 in plate 12 to an electron detector shown at 16 to the right of FIG. 1. These detectors may be of any conventional type well known to the art.

The apertured metal plates 11 and 12 are suitably biased with respect to the assumed ground potential of the ionization region 10 by any suitable power source. The power source shown in the drawings includes a battery 17 having its midpoint grounded and connected across a multi-tap potentiometer 18, whose midpoint is also grounded. One movable tap of the potentiometer 19 is connected to plate 11 while another movable tap 20 is connected directly to plate 12. As a result, the plates 11 and 12 will be respectively at negative and positive potentials with respect to ground. If the distances between the center line 21 of the ionization region 10 and the plates 11 and 12 are equal, the voltages between tap 19 and ground and tap 20 and ground will also be equal. At any rate, whether or not these distances are equal, the field intensities between the source and each of the plates should be the same.

A relatively higher field intensity for acceleration of the positive'ions is provided in a region more remote from the ionization region by the first metal plate 22 of an immersion or unipotential lens generally indicated at 23' and which will be described more fully hereinafter. The plate 22 has an aperture 24 therethrough which is aligned with the aperture 13 in plate 11. The plate 22 is also connected to the movable tap 25 of the potentiometer 19, and the ratio of the voltage difference between the plates 22 and 11 to the distance therebetween is much greater than the ratio ofthe voltage difference between plate 11 and the source to the distance between that plate and the center line 21. As a result, thepositive ions from ionization region 10 are exposed first to a relatively low field intensity and then to a higher field intensity.

' If there were no other lens plates between plate 11 and plate 22, the eifect of this construction and the relative voltage levels indicated would be to converge the ions from the source 10 as they pass through the aperture 13 in plate 11 to a focus a short distance to the left of that aperture. The, ions would then diverge again in a cone of relatively large apex angle such that a large number of. such ions would strike plate 22 and would therefore not reach the detector. 7

In order to prevent this effect, the apparatus is provided with an additional apertured metal plate 26' between the plate 11 and the plate 22. This plate is appropriately biased negatively with respect to the plate 11 by connection to a movable tap 27 of the potentiometer 18; The voltage level is chosen in correspondence with the distances involved such that the ratio of the voltage difference between plate 26 and plate 11 to the distance therebetween is less than the ratio of the voltage difference 7 between plate 11 and the source to. the distance between the plate and the center line 21. The aperture lens 26,

therefore exerts a slightly converging effect, while the apertured plate 11 exerts a diverging eifect. The overall effect is to direct the ions through the aperture 28 in plate 26 in slightly divergent form such that the ions appear to originate in a virtualfocus well to the right of the ionization region 10. V

The unipotential or immersion lens 23 includes, in addition to. the plate 22, additional apertured metal plates 29 and 39. The plates 22 and 39 are connected directly together to. the movable tap of the potentiometer. The plate 29, however, is connected to a tap'21 :which, as

shown, is positioned such that thenegative voltage on 15. The driftitube may be appropriately of metal and connected to the plate 30 so that there is no appreciable electrical field in at least a major portion of that structure. It Will be understood,'however, that the field developed by the lens 23 will penetrate into the drift tube to some extent. Further, if such techniques or electron multiplicationare employed in the ion detector 15, the field therefrom may'also penetrate into the drift tube. Neverthelessflhe drift tube, or at thereof, is substantially field-free.

0 Selecting the voltage levels for the desired focusing 7 action, the voltages on plates '22 and 26 are adjusted to levels such that the virtual focus of the ions is the same distance to the right of, plate 29 as the detector 15: is to the left thereof.. The focallength of the immersion lens 23 can then be adjusted by control of the voltage on plate 29 suchthat it is substantially identical with the distance between plate 22 and detector 15. ,If such is done, the

least a major portiondivergent ion beam will be translated into a parallel beam directed to the detector 15.

It will be seen that the aperture 24 in the plate 22 is approximately the same size as the aperture -36 in the plate 30. The aperture 37 in the plate 29- is of somewhat larger size, because the focusing action of the unipolar 0r immersion lens 23 is such as to first diverge the beam and then converge it, and it is desirable that no ions strike the edges of the aperture 37.

In fact, it will be noted that the aperture 13 in plate 11 and the corresponding aperture 14 in plate 12 are of the smallest sizes of any apertures in the various focusing plates. These apertures determine the size of the respective ion and electron beams to be directed to the'detectors, and the apertures in the other plates of the system are sufficiently larger than such apertures that the beams will not strike the edges of the plates. This is desirable both in order that the number of ions and electrons in the beam not be reduced, and also in order that any secondary effects such as generation of secondary electrons may be avoided. The apertures in all the plates are aligned along a straight line between the source and the detector and are appropriately of circular shape. I

The electron beam portion of the accelerator apparatus further includes a plate 33 having an aperture 39 therethrough. The plate 38 functions similarly to the plate 26 of the ion accelerator and is supplied with biasing voltage appropriately of the same order of magnitude as on the plate 26 by connection to a movable tap 40 on potentiometer 18.

The operation of the apparatus for acceleration and focusing of the respective positive ion and electron beams will be apparent from the above description.

However, the theory: of operation may be more easily understood by analogy of the well-known Galilean telescope in which light rays from a distant object are first converged by an object lens and then diverged by an ocular. The ocular is spaced from the objective by a distance equal to the difference in their focal lengths, the result being that the convergent rays are changed into parallel rays at the eyepiece.

In the present invention, the action is the reverse of that of the Galilean telescope in that the ions from the closely spaced source are first diverged and then converged to form a parallel beam directed to a remote ion detector.

Solely in order to furnish an illustration of the voltages and plate spacings which might be used with an apparatus designed in accordance with the invention, and not for the purpose of limiting the invention in any way, the followingillustrations are given:

V11=V12=800 Volts V2 =V3 =15OO Volts V22 V30=-5000 VOliS V29=4000 volts (It will be evident from the above that the voltages on the plates 11 and 26 are negative, while the voltages on plates 12 and 28 arepositive with respect to the source voltage.)

S /2 inch S /2 inch S =1 /2 inches S =inch S =4O inches tion in the illustrated embodiment without departure from the scope of the invention. For instance, the immersion lens 23 is not itself critical to the invention. Rather another suitable convergent lens, such as a pair of quadripole lenses, could be employed. Therefore, the invention is to be considered limited only by the appended claims.

We claim:

1. In an ion accelerator position between a source of ions and an ion detector,

divergent lens means positioned adjacent the source and including means for establishing an electrostatic field urging the ions toward the detector, said divergent lens means being operable to slightly diverge ions from the source to cause them to appear to have originated at a virtual source farther from the detector than the actual source,

and a convergent lens positioned between the divergent lens and the detector and including means for establishing a second electrostatic field of higher intensity urging the ions toward the detector,

the distance between the two lenses and the voltage difference therebetween being such as to change the divergent ion beam into a substantially parallel beam as it passes through said convergent lens,

the convergent lens being separated from the ion detector by a field-free drift region. 2. The apparatus of claim 1 in which said divergent lens includes a first and a second plate positioned in that order between the source and the detector, said plates having aligned apertures through which the ion beam passes,

and means for supplying voltages to said plates of magnitudes such that the ratio of the voltage difference between the first plate and the source to the distance therebetween is greater than the ratio of the voltage difference between the first and second plates to the distance therebetween.

3. The apparatus of claim 2 in which said convergent lens includes a plurality of plates having aligned apertures through which the ion beam may pass, the apertures in all of said plurality of plates and said second plate being sufficiently larger than the aperture in said first plate that ions passing through said first plate do not impinge on any of the other plates.

4. The apparatus of claim 1 in which said divergent lens comprises i a first and a second lens portion positioned in that order between said source and said convergent lens, said first lens portion being divergent in operation and said second portion being convergent in operation but to a lesser extent than said first portion is divergent, the combination of said first and second lens portions being operable to direct a beam which is only slightly divergent toward said convergent lens.

5. In an ion accelerator for positive ions positioned between a source thereof and a positive ion detector,

a first pair of metal plates positioned at opposite sides of the source, a third metal plate positioned between said first pair of plates and the ion detector,

means for supplying opposite polarity voltages to said first pair of plates with the one plate thereof nearest the ion detector biased negatively,

means for biasing said third plate negatively with respect to said one plate at a voltage such that the ratio of the voltage difierence between said one plate and the source to the spacing therebetween is greater than the ratio of the voltage difference between said one and third plates to the distance therebetween,

a convergent ion lens positioned between said third plate and the detector and including a fourth metal plate,

means for biasing said fourth plate negatively with respect to said third plate at a voltage such that the ratio of the vo tage difference between said fourth and third plates to the distance therebetween is much greater than the ratio of the voltage difference between said one plate and the source to the distance therebetween,

all of said plates having apentures therethrough aligned along a path between the source and the detector along which the ions pass,

and a substantially field-free drift tube between said convergent lens and the detector.

6. The apparatus of claim 5 in which said convergent lens is a unipotential lens including said fourth plate and fifth and sixth apertured metal plates positioned in that order between the third plate and said drift tube with apertures aligned to permit ions to pass into the drift tube,

and means for biasing the sixth plate to the same voltage as the fourth plate and the fifth plate to a different voltage,

such drift tube being of metal and having its axis aligned with the ion path and being at substantially the same voltage as said fourth and sixth plates.

7. The apparatus of claim 6 including means for adjusting the various voltages on the metal plates so that the virtual focus from which the ions appear to originate is at substantially the same distance to one side of said fifth plate as the ion detector is to the other side thereof and that the focal length of said convergent lens is substantially the same distance.

8. The apparatus of claim 5 including means for adjusting the various voltages on the metal plates so that the virtual focus from which the ions appear to originate is at substantially the same distance to one side of said convergent lens as the detector is to the other side thereof,

and means for adjusting the focal length of said convergent lens to substantially the same distance.

9. For use in a coincidence mass spectrometer including an electron detector and a positive ion detector positioned at opposite sides of an ion source, an accelerator comprising first and second apertured metal plates positioned at opposite sides of said source at substantially the same distances therefrom,

third and fourth apertured metal plates positioned at opposite sides of the source respectively between the first plate and the ion detector and between the second plate and the electron detector and at substantially the same distances from the source,

means for biasing the first and third plates negatively and the second and fourth plates positively with respect to said source at such voltages that the ratio of the voltage difference between the first plate and the source to the distance therebetween and the ratio of the voltage difference between the second plate and the source to the distance therebetween are re spectively greater than the ratio of the voltage difference between the third and first plates to the distance therebetween and the ratio of the voltage difference between the fourth and second plates to the distance therebetween, the fourth and third plates being respectively biased positively and negatively with respect to the second and first plates,

a convergent ion lens positioned between said third plate and said ion detector and including a fifth apertured metal plate,

means for biasing the fifth plate negatively with respect to said third plate at a voltage such that the ratio of the voltage difierence therebetween to the distance therebetween is much greater than the ratio of the voltage difference between the first plate and the ion source to the distance therebetween,

said several biasing means including means for adjusting the voltages on the plates so that the virtual focus from which the positive ions appear to originate is at substantially the same distance to one side of the convergent lens as the ion detector is to the other" side thereof,

means for adjusting the focus of said convergent lens to substantially the same distance,

and a metal drift tube between said convergent lens and the ion detector biased to substantially the same voltage as said fifth plate,

the apertures through the several plates and the axis of the drift tube being aligned along a straight line between the electron detector and the ,ion detector with the apertures through the first and second plates defining the sizes of the respective ion and electron beams and the apertures through the other plates sufficiently larger that the electrons and ions do not impinge upon their plates.

10. The apparatus of claim 9 in which said convergent lens is of the unipotential type including said fifth plate and sixth and seventh apertured metal plates positioned in that order between said third plate and the ion detector, with their apertures aligned with the apertures through the other plates and at least as large as the aperture through saidtfifth plate, 7

References Cited in the file of this patent UNITED STATES PATENTS Coggeshall et a1 Dec. 28, 1948 Robinson Mar. 5, 1957 OTHER REFERENCES An Ion Velocition, by A. E. Cameron et al. from 15 ,The Review of Scientific Instruments, volume 19, No.

'9, September 1948, pages 605 to 607.

Ion Source for Mass Spectrography, by R. F. K. Herzog et al. from The Physical Review, volume 76,

0 1949, pages 855 and 856. 

1. IN AN ION ACCELERATOR POSITION BETWEEN A SOURCE OF IONS AND AN ION DETECTOR, DIVERGENT LENS MEANS POSITIONED ADJACENT THE SOURCE AND INCLUDING MEANS FOR ESTABLISHING A ELECTROSTATIC FIELD URGING THE IONS TOWARD THE DETECTOR, SAID DIVERGENT LENS MEANS BEING OPERABLE TO SLIGHTLY DIVERGE IONS FROM THE SOURCE TO CAUSE THEM TO APPEAR TO HAVE ORIGINATED AT A VIRTUAL SOURCE FARTHER FROM THE DETECTOR THAN THE ACTUAL SOURCE, AND A CONVERGENT LENS POSITIONED BETWEEN THE DIVERGENT LENS AND THE DETECTOR AND INCLUDING MEANS FOR ESTABLISHING A SECOND ELECTROSTATIC FIELD OF HIGHER INTENSITY URGING THE IONS TOWARD THE DETECTOR, 